I take your point, though there are many application not following this rule and pressing back just quits these apps instead of moving them to background. Even some Google apps behaves like this, e.g. Maps or Sky Map. So it definitely is some issue related to how Unity works (or maybe Mono).

Anyway - what should be the proper way of handling back button then? I.e. I launch a game, press back and want to be taken out of the game, which should be in the background now. As far as I know, there is no such possibility in Unity SDK. Or hopefully I'm missing something...

Processes will be quit, whereas apps should not. I know there are games that dont follow this rule, and that may be what you want to do..not sure if your app will be rejected for hard quitting your app tho(i know iphone has strict rules about not using this functionality).

Heres a portion of an article explaining why killing an app isnt exactly doing any good:

"It's actually possible that this will worsen your phone's performance and battery life. Whether you're manually killing apps all the time or telling the task killer to aggressively remove apps from your memory, you're actually using CPU cycles when you otherwise wouldn't—killing apps that aren't doing anything in the first place."
Android Task Killers

As far as i know there is no ReturnToHomepage type functionality. You can write a plugin that does this using the following native code

Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_HOME);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(intent);

Thank you for all the information and for providing the snippet. Now I have to consider which path to follow, but I guess I'll stick to Application.Quit() - if Angry Birds and Temple Run (made in Unity) can successfully use it, then I believe I can too ;)